1. Field of the Invention
This invention relates generally to methods and apparatus for producing plasma; in particular, the invention relates to an electrode for establishing a steady-state glow-discharge plasma at atmospheric pressure and low temperatures.
2. Description of the Related Art
Plasma is an ionized form of gas that can be obtained by ionizing a gas or liquid medium using an AC or DC power source. A plasma, commonly referred to as the fourth state of matter, is an ensemble of randomly moving charged particles with sufficient density to remain, on average, electrically neutral. Plasmas are used in very diverse processing applications, ranging from the manufacture of integrated circuits for the microelectronics industry, to the treatment of fabric and the destruction of toxic wastes.
In particular, plasmas are widely used for the treatment of organic and inorganic surfaces to promote adhesion between various materials. For example, polymers that have chemically inert surfaces with low surface energies do not allow good bonding with coatings and adhesives. Thus, these surfaces need to be treated in some way, such as by chemical treatment, corona treatment, flame treatment, and vacuum plasma treatment, to make them receptive to bonding with other substrates, coatings, adhesives and printing inks. Corona discharge, physical sputtering, plasma etching, reactive ion etching, sputter deposition, plasma-enhanced chemical vapor deposition, ashing, ion plating, reactive sputter deposition, and a range of ion beam-based techniques, all rely on the formation and properties of plasmas.
Corona discharges are widely used in particular for treating plastic films, foils, papers, etc. to promote adhesion with other materials by increasing the surface energy of the film. A corona discharge is established between two electrodes by applying a high voltage to one of the electrodes while the other is connected to ground at typical frequencies in the order of 10-50 kHz. These conditions produce locally concentrated discharges known in the art as streamers, which lead to some non-uniformity in the treatment of film surfaces and can also damage the film by producing low molecular weight species that adversely affect adhesion to the surface. Furthermore, the streamers of corona treatment can produce backside effects on the film being treated, which is undesirable in many applications. Nevertheless, corona treatment is extensively used in the industry for improving the surface energy of materials.
Glow-discharge plasma treatment is also an effective method of treating surfaces to increase their wettability and adhesion to various materials. Glow discharge provides a more uniform and homogenous plasma that produces a more consistent surface treatment than corona treatment, thereby avoiding unintentional back treatment of the film. Glow-discharge plasma is characterized by high-energy electrons that collide with, dissociate and ionize low-temperature neutrals, creating highly reactive free radicals and ions. These reactive species enable many chemical processes to occur with otherwise unreactive low-temperature feed stock and substrates. Based on these properties, low-density glow-discharge plasmas are usually utilized for low material-throughput processes involving surface modification. These plasmas are typically formed by partially ionizing a gas at a pressure well below atmosphere. For the most part, these plasmas are weakly ionized, with an ionization fraction of 10xe2x88x925 to 10xe2x88x921, established with AC or DC power in systems with varied geometries. These systems always require vacuum chambers and pumps to maintain a low pressure, which increases operating costs and maintenance.
There has been an extensive effort to develop plasma systems capable of operating at atmospheric pressure for surface treatment of polymer films, foils, and paper, in order to avoid capital and maintenance expenditures for vacuum chambers and pumps. It is known that atmospheric plasma can be generated at relatively low temperatures with a proper power source, the insertion of a dielectric layer between the electrodes, and the use of an appropriate gas mixture as plasma medium. For surface treatment of polymer films, fabrics, paper, etc., atmospheric plasma can be established between two electrodes using an inert gas such as helium under particular operating conditions. Usually one electrode is attached to a high voltage power supply, and a rotating drum is grounded and acts as the other electrode. One electrode is coated with a ceramic layer and the plasma gas is injected between electrodes.
Examples of glow-discharge plasma systems operating at atmospheric pressure are described in U.S. Pat. Nos. 5,387,842, 5,403,453, 5,414,324, 5,456,972, 5,558,843, 5,669,583, 5,714,308, 5,767,469, and 5,789,145. Co-owned U.S. Pat. No. 6,118,218, incorporated herein by reference, disclosed a plasma treatment system capable of producing a steady glow discharge at atmospheric pressure with different gas mixtures operating at frequencies as low as 60 Hz. The invention consists of incorporating a porous metallic layer in one of the electrodes of a plasma treatment system. A plasma gas is injected into the electrode at substantially atmospheric pressure and allowed to diffuse through the porous layer, thereby forming a uniform glow-discharge plasma. As in prior-art devices, the film material to be treated is exposed to the plasma created between this electrode and a second electrode covered by a dielectric layer. Because of the micron size of the pores of the porous metal, each pore also produces a hollow cathode effect that facilitates the ionization of the plasma gas. As a result, a steady-state glow-discharge plasma is produced at atmospheric pressure and at power frequencies as low as 60 Hz. The inventors discovered that, in order for the electrode holes to operate effectively for producing an optimal glow discharge, their size must approach the mean free path of the plasma gas at the system""s operating pressure.
Thus, the ability to produce a reliable and uniform glow-discharge plasma at atmospheric pressure has greatly improved the flexibility of operation of plasma treatment processes, but a serious constraint remains as a result of the required geometry of the apparatus. As illustrated in FIG. 1, conventional plasma treatment systems consists substantially of a plasma treater 10 mounted on a roller 12. A film 14 of material to be treated is passed through the system between the plasma treater and the roller. The roller 12 is grounded and coated with a dielectric material 16, such as polyethylene teraphthalate (PET). The plasma treater 10 contains at least one electrode which is connected, through a cable 18, to an AC power supply 20. The treater is held in place by a holding bracket 22 designed to maintain a distance of 1-2 mm between the roller 12 and the treater 10. This distance, which varies with operating conditions, plasma medium composition, and electrode configuration, is important in establishing a steady plasma flow; therefore, it is very desirable to maintain a gap determined to be optimal. Plasma gas, such as helium, argon, and mixtures of an inert gas with nitrogen, oxygen, air, carbon dioxide, methane, acetylene, propane, ammonia, or mixtures thereof, are used to sustain a uniform and steady plasma. The gas is supplied to the treater 10 through a manifold 24 that feeds the electrode of the apparatus.
As a result of this conventional configuration of plasma treaters, the film substrate 14 being treated is necessarily always bound by the two electrodes of the system (that is, the plasma electrode and the grounded roller). Since plasma formation is achieved only within a limited range of spacing between the two electrodes, the substrate to be treated is obviously also limited in thickness and shape. Therefore, it would be very desirable to have plasma treatment apparatus capable of treating the surface of substrates regardless of their thickness and shape. The present invention relates to an improved electrode for operation under such advantageous conditions.
The primary objective of this invention is an electrode assembly that can be operated to produce plasma without extending the electric field through the substrate material being treated, thereby making it possible to treat thicker material than ever before possible.
Another goal is an electrode assembly that can be used to treat substrates without limitations based on shape or chemical composition of the substrate.
Another objective is also an electrode assembly that makes it possible to treat a particulate substrate three dimensionally.
Another goal is an atmospheric-pressure glow-discharge plasma system operable under steady-state conditions at a relatively low temperature.
Yet another objective is a plasma-treatment process that can be carried out using a power system operating at relatively low frequencies.
Still another objective is an electrode assembly that is suitable for incorporation with existing plasma equipment.
Another goal is also a device suitable for combining plasma treatment with vapor deposition at atmospheric pressure.
A final objective is a plasma-treatment process that can be implemented easily and economically according to the above stated criteria.
Therefore, according to these and other objectives, the preferred embodiment of the invention consists of two metallic electrodes embedded side by side in a dielectric medium having a porous outer layer defining an exposed treatment space. One of the electrodes is made of a porous metal and serves as a conduit for introducing a plasma gas into the treatment system. The plasma gas is injected into the porous electrode at substantially atmospheric pressure and allowed to diffuse through the porous electrode and the porous dielectric layer into the exposed treatment space. The two electrodes are energizes in conventional manner, using one of the electrodes as a ground, to create an electric field between them and produce a uniform glow-discharge plasma in the treatment space. The material to be treated can be exposed to the plasma so created without substantial limitation as to thickness, geometry and composition. By eliminating the need to maintain an electric field across the substrate being treated, the electrode assembly of the invention makes it possible to treat thick substrates and substrates of metallic composition that cannot be treated with prior-art equipment. In addition, powdery substrates can be treated by adding a shaker to a belt used to convey them through the plasma field.
According to another embodiment of the invention, a conventional electrode is coupled to the porous electrode through a dielectric partition in a compact assembly to produce an electric field across a treatment space adjacent to an exterior surface of the assembly. The assembly is energized in conventional manner using one of the electrodes as the ground. When a plasma gas is diffused through the porous electrode, a steady-state glow-discharge plasma is produced in the treatment space extending through the electric field and can be similarly used advantageously to treat a substrate exposed to the electrode assembly.
According to yet another aspect of the invention, vapor deposition is carried out in combination with plasma treatment by vaporizing a substance of interest, mixing it with the plasma gas, and diffusing the mixture through the porous electrode. A heater is used to maintain the temperature of the electrode above the condensation temperature of the substance to prevent deposition during diffusion. Thus, plasma treatment and vapor deposition can be carried out on a target substrate at the same time at atmospheric pressure.
Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiment and particularly pointed out in the claims. However, such drawings and description disclose only some of the various ways in which the invention may be practiced.